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Scintillators with potential to supersede lanthanum bromide

Description: New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high light yield materials: Europium-doped alkaline earth halides and Cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method - SrI{sub 2}, CaI{sub 2}, SrBr{sub 2}, BaI{sub 2} and BaBr{sub 2} - SrI{sub 2} is the most promising. SrI{sub 2}(Eu) emits into the Eu{sup 2+} band, centered at 435 nm, with a decay time of 1.2 {micro}s and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramics fabrication allows production of Gadolinium- and Terbium-based garnets which are not growable by melt techniques due to phase instabilities. While scintillation light yields of Cerium-doped ceramic garnets are high, light yield non-proportionality and slow decay components appear to limit their prospects for high energy resolution. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance energy resolution.
Date: June 1, 2009
Creator: Cherepy, Nerine; Payne, Steven; Aszatlos, Steve; Hull, Giulia; Kuntz, J.; Niedermayr, Tom et al.
Partner: UNT Libraries Government Documents Department

Distribution of zinc, resistivity, and photosensitivity in a Vertical Bridgman grown Cd1-xZnxTe ingot.

Description: We present the results of a comprehensive study of distribution of zinc, resistivity, and photosensitivity in a Cd{sub 1-x}Zn{sub x}Te ingot grown by the Vertical Bridgman method. We used several complementary methods, viz., glow discharge mass spectroscopy, photoluminescence-, resistivity-, and photosensitivity-mapping, along with photo-induced current transient spectroscopy to characterize the material. We identified electronic levels in the band-gap responsible for compensation, recombination, and photosensitivity.
Date: May 1, 2008
Creator: Babentsov, V.; Franc, J.; Fauler, A.; Fiederle, M. & James, R. B.
Partner: UNT Libraries Government Documents Department

Tellurium precipitates in (Cd,Mn)Te:V crystals: Effects of annealing

Description: We suggest that (Cd,Mn)Te is a suitable material for fabricating gamma- and X-ray detectors. Our investigations, reported here, are focused on producing high-quality (Cd,Mn)Te crystals with high resistivity (10{sup 9} {Omega}-cm) by the Bridgman method. As-grown, undoped (Cd,Mn)Te crystals are typically p-type, signifying that they contain excess Cd vacancies (acting as acceptors), accumulated during growth. Doping with vanadium atoms, which function as compensating centers, results in a semi-insulating material (Cd,Mn)Te:V. Properly annealing the platelets in cadmium vapors at uniform temperature reduces the number of cadmium vacancies, and lowers the level of the vanadium doping required for compensation. We found that annealing in cadmium vapors not only decreases the concentration of the native cadmium vacancies but also improves the crystal's quality. Infrared observations of the interior of the samples show that annealing in a temperature gradient perpendicular to the platelet has an additional effect, viz., the tellurium precipitates migrate towards the side where the temperature is higher. We demonstrate, with IR pictures of monocrystalline (Cd,Mn)Te:V platelets cut parallel to the (111) crystal planes, the influence on tellurium inclusions and precipitates of various conditions of annealing in cadmium vapors.
Date: October 19, 2008
Creator: Kochanowska,D.; Mycielski, A.; Witkowska-Baran, M.; Szadkowski, A.; Witkowska, B.; Kaliszek, W. et al.
Partner: UNT Libraries Government Documents Department

Bridgman growth and characterization of bulk single crystals of Ga{sub 1{minus}x}In{sub x}Sb for thermophotovoltaic applications

Description: Thermophotovoltaic generation of electricity is attracting renewed attention due to recent advances in low bandgap (0.5--0.7 eV) III-V semiconductors. The use of mixed pseudo-binary compounds allows for the tailoring of the lattice parameter and the bandgap of the material. Conventional deposition techniques (i.e., epitaxy) for producing such ternary or quaternary materials are typically slow and expensive. Production of bulk single crystals of ternary materials, for example Ga{sub 1{minus}x}In{sub x}Sb, is expected to dramatically reduce such material costs. Bulk single crystals of Ga{sub 1{minus}x}In{sub x}Sb have been prepared using a Bridgman technique in a two-zone furnace. These crystals are 19 mm in diameter by approximately 50 mm long and were produced using seeds of the same diameter. The effects of growth rate and starting materials on the composition and quality of these crystals will be discussed and compared with other attempts to produce single crystals of this material.
Date: December 1, 1997
Creator: Boyer, J.R. & Haines, W.T.
Partner: UNT Libraries Government Documents Department

Bulk growth of GaSb and Ga{sub 1{minus}x}In{sub x}Sb

Description: GaSb and InGaSb have been demonstrated to be suitable choices for high efficiency thermophotovoltaic (TPV) cells. Synthesis and growth of bulk GaSb single crystals and GaInSb polycrystals have been carried out by the vertical Bridgman technique, with a baffle immersed in the melt and by complete encapsulation of the melt by low melting temperature alkali halides or oxides. The critical roles of the baffle and the encapsulation are discussed. Efforts in obtaining device grade GaSb with superior structural and electrical properties and compositionally homogeneous GaInSb are described, emphasizing the key steps in the growth cycle developed to obtain good crystalline quality.
Date: May 1, 1997
Creator: Dutta, P.S.; Ostrogorsky, A.G. & Gutmann, R.J.
Partner: UNT Libraries Government Documents Department

Correlation between nuclear response and defects in CZT

Description: Vertical high pressure Bridgman (VHPB) was considered until now to be the most successful crystal growth method to produce Cd{sub 1{minus}x}Zn{sub x}Te (CZT), (0.04 < x < 0.24), for X- and gamma-ray detector crystals. Recently Horizontal Bridgman (HB) Cd{sub 1{minus}x}Zn{sub x}Te crystals produced by IMARAD Co. have also been successfully fabricated into nuclear spectroscopic radiation detectors. In view of the database of many years' study of the electrical properties of VHPB CZT grown and obtained from various sources, the authors also studied the HB CZT crystals in order to compare the defects present in both different kinds of crystals grown by different methods. The VHB-grown samples were examined using thermoelectric emission spectroscopy (TEES), X- and gamma ray spectroscopy and laser induced transient charge technique (TCT). The surface and the bulk crystalline homogeneity were mapped using triaxial double crystal x-ray diffraction (TADXRD) and infrared transmission spectroscopy (IR). They have found a correlation between crystallinity, IR transmission microstructure and trapping times. Spectrometer grade VHPB CZT crystals exhibit trapping times of 20 {micro}s for electrons and 7 {micro}s for holes, however, regions, which were opaque to IR transmission, had trapping times shorter by one order of magnitude. The trapping times of HB CZT for electrons, were 10--15 {micro}s. A similar trend has been observed on VHPB CZT crystals with poor crystallinity. The HB CZT crystals that they measured in this study had a crystallinity that was inferior to that of the best spectroscopic grade VHPB crystals.
Date: July 19, 1999
Creator: Hermon, H.; Schieber, M.; James, R. B.; Lee, E.; Cross, E.; Goorsky, M. et al.
Partner: UNT Libraries Government Documents Department

Analysis of CZT crystals and detectors grown in Russia and the Ukraine by high-pressure Bridgman methods

Description: Sandia National Laboratories (SNL) is leading an effort to evaluate vertical high pressure Bridgman (VHPB) Cd{sub 1-x}Zn{sub x}Te (CZT) crystals grown in the former Soviet Union (FSU) (Ukraine and Russia), in order to study the parameters limiting the crystal quality and the radiation detector performance. The stoichiometry of the CZT crystals, with 0.04 < x < 0.25, has been determined by methods such as proton-induced X-ray emission (PIXE), X-ray diffraction (XRD), microprobe analysis and laser ablation ICP mass spectroscopy (LA-ICP/MS). Other methods such as triaxial double crystal x-ray diffraction (TADXRD), infrared transmission spectroscopy (IR), atomic force microscopy (AFM), thermoelectric emission spectroscopy (TEES) and laser induced transient charge technique (TCT) were also used to evaluate the material properties. The authors have measured the zinc distribution in a CZT ingot along the axial direction and also its homogeneity. The (Cd+Zn)/Te average ratio measured on the Ukraine crystals was 1.2, compared to the ratio of 0.9-1.06 on the Russian ingots. The IR transmission showed highly decorated grain boundaries with precipitates and hollow bubbles. Microprobe elemental analysis and LA-ICP/MS showed carbon precipitates in the CZT bulk and carbon deposits along grain boundaries. The higher concentration of impurities and the imperfect crystallinity lead to shorter electron and hole lifetimes in the range of 0.5--2 {micro}s and 0.1 {micro}s respectively, compared to 3--20 {micro}s and 1--7 {micro}s measured on US spectrometer grade CZT detectors. These results are consistent with the lower resistivity and worse crystalline perfection of these crystals, compared to US grown CZT. However, recently grown CZT from FSU exhibited better detector performance and good response to alpha particles.
Date: January 10, 2000
Creator: Hermon, H.; Schieber, M.; James, R. B.; Lee, E. Y.; Yang, N.; Antolak, A. J. et al.
Partner: UNT Libraries Government Documents Department

Analysis of grain boundaries, twin boundaries, and Te precipitates in CdZnTe grown by high-pressure Bridgeman method

Description: Grain boundaries and twin boundaries in commercial Cd{sub 1{minus}x}Zn{sub x}Te, which is prepared by a high pressure Bridgeman technique, have been investigated with transmission electron microscopy, scanning electron microscopy, infrared light microscopy and visible light microscopy. Boundaries inside these materials were found to be decorated with Te precipitates. The shape and local density of the precipitates were found to depend on the particular boundary. For precipitates that decorate grain boundaries, their microstructure was found to consist of a single, saucer shaped grain of hexagonal Te (space group P3{sub 1}21). Analysis of a Te precipitate precipitates by selected area diffraction revealed the Te to be aligned with the surrounding Cd{sub 1{minus}x}Zn{sub x}Te grains. This alignment was found to match the (111) Cd{sub 1{minus}x}Z{sub x}Te planes with the (1{bar 1}01) planes of hexagonal Te. Crystallographic alignments between the Cd{sub 1{minus}x}Zn{sub x}Te grains were also observed for a high angle grain boundary. The structure of the grain boundaries and the Te/Cd{sub 1{minus}x}Zn{sub x}Te interface are discussed.
Date: March 1, 1998
Creator: Heffelfinger, J.R.; Medlin, D.L. & James, R.B.
Partner: UNT Libraries Government Documents Department

Evaluation of CZT crystals from the former Soviet Union

Description: Vertical high pressure Bridgman (VHPB) Cd{sub 1{minus}x}Zn{sub x}Te (0.04 < x < 0.24) detector crystals grown in the Ukraine and Russia have been evaluated and compared to US-grown materials. Various analytical techniques were used to study the materials for trace impurities, precipitates, crystallinity, and electrical transport properties. Relatively high concentrations of carbon and trace impurities such as Se, Nd and Si have been detected in the crystals. In most cases, the crystals showed lower resistivity than US-grown CZT. However, recent crystals grown in Russia exhibited better detector performance than those grown in prior years, and good response to an {sup 241}Am radioactive source was found. Electron lifetimes below 1 {micro}s were measured in crystals having significant numbers of micro-defects, compared to lifetimes of 5--15 {micro}s found in spectrometer grade materials produced in the US. Furthermore, the zinc composition along the growth axis showed better homogeneity in comparison with the US material.
Date: January 26, 1998
Creator: Hermon, H.; Schieber, M.; James, R. B.; Antolak, A. J.; Morse, D. H.; Brunett, B. et al.
Partner: UNT Libraries Government Documents Department

IMPROVEMENT OF CdMnTe DETECTOR PERFORMANCE BY MnTe PURIFICATION

Description: Residual impurities in manganese (Mn) are a big obstacle to obtaining high-performance CdMnTe (CMT) X-ray and gamma-ray detectors. Generally, the zone-refining method is an effective way to improve the material's purity. In this work, we purified the MnTe compounds combining the zone-refining method with molten Te, which has a very high solubility for most impurities. We confirmed the improved purity of the material by glow-discharge mass spectrometry (GDMS). We also found that CMT crystals from a multiply-refined MnTe source, grown by the vertical Bridgman method, yielded better performing detectors.
Date: April 25, 2011
Creator: Kim, K. H.; Bolotnikov, A. E.; Camarda, G. S.; Tappero, R.; Hossain, A.; Cui, Y. et al.
Partner: UNT Libraries Government Documents Department

(Cd,Mn)Te detectors for characterization of x-ray emissions generated during laser-driven fusion experiments

Description: We present our measurements of (Cd,Mn)Te photoconductive detectors (PCDs), fabricated for the goal of measuring both the temporal and spectral dependences of X-ray emissions generated from laser-illuminated targets during the inertial confinement fusion experiments. Our Cd{sub 1-x}Mn{sub x}Te (x = 0.05) single crystals, doped with V, were grown using a vertical Bridgman method and, subsequently, annealed in Cd for the highest resistivity ({approx}10{sup 10} {Omega}cm) and a good mobility-lifetime product ({approx}10{sup -3} cm{sup 2}/V). The 1-mm- and 2.3-mm-thick detectors were placed in the same housing as two 1-mm-thick diamond PCDs. All devices were pre-screened by a 7.6-mm-thick Be X-ray filter with a frequency cutoff of 1 keV. The incident shots from the OMEGA laser were 1-ns-long square pulses with energies ranging from 2.3 kJ to 22.6 kJ, and the PCDs were biased with 5000 V/cm. The response amplitudes and rise times of our (Cd,Mn)Te PCDs were comparable with the diamond detector performance, while the decay times were 4 to 10 times longer and in the 2-5 ns range. We observed two X-ray emission events separated by 1.24 ns. The first was identified as caused by heating of the target and creating a hot corona, while the second one was from the resulting compressed core. For comparison purposes, our testing was performed using {approx}1 keV X-ray photons, optimal for the diamond PCD. According to the presented simulations, however, at X-ray energies >10 keV diamond absorption efficiency drops to <50%, whereas for (Cd,Mn)Te the drop occurs at {approx}100 keV with near perfect, 100% absorption, up to 50 keV.
Date: October 19, 2008
Creator: Cross, A. S.; Knauer, J. P.; Mycielski, A.; Kochanowska, D.; Wiktowska-Baran, M.; Jakiela, R. et al.
Partner: UNT Libraries Government Documents Department

Closed loop control techniques for the growth of single crystal turbine components

Description: Analysis of processes used for the production of single crystal turbine components reveals significant shortcomings. Inadequate consideration has been made of the fact the system is cooling dominated and that the amount of cooling tends to increase as the emissive cooling area expands during the process. Experimental evidence suggests that during processing, this increased cooling causes the solidification interface to move away from the baffle and become curved. The motion of the interface results in a decrease in the solidification gradient. The combination of these actions can result in variations in PDAS (primary dendrite arm spacing), grain misalignment and the production of defects. It is shown that despite this tendency, microstructural stabilization may be achieved through the use of the heat of fusion as an internal process heat source.
Date: December 31, 1995
Creator: Schlienger, M.E.
Partner: UNT Libraries Government Documents Department

AlSb photonic detectors for gamma-ray spectroscopy. Progress report, October 1994--August 1995

Description: Aluminum antimony (AlSb) is an indirect band gap semiconductor with Eg of about 1.62 eV at 300 K and about 1.75 eV at 77 K. This material, is extremely difficult to obtain in single crystal form because of the very high reactivity of aluminum with oxygen, and the high volatility of antimony. Moreover, molten AlSb reacts with nearly all crucible materials available. Since Welker`s first attempts in 1952, only very few different experimental approaches have been used to grow single crystals of AlSb, e.g. by Bridgman, Czochralski and MBE. All experimental results, however, indicate that many of the properties of AlSb, e.g. carrier concentration, electron-hole mobility and carrier life-time, differ significantly from the theoretically predicted values. The main objective of this research period has been to develop a method leading to improved crystallographic and electronic quality of AlSb crystals, making them more suitable for device applications. The research program was aimed along the following two directions: (1) study the growth of AlSb via Bridgman, Czochralski and THM techniques; (2) comprehensive characterization of grown material, related to the use of compounds for high energy gamma detectors. Variables in the growth study were growth temperature, equilibrium pressure, growth rate, doping, crucible material, seeding and encapsulation. The characterization study included crystallographic quality (grain size, etch pits, precipitates, inclusions), electronic quality (conductivity type, carrier concentration and mobility), optical properties (spectral absorption, photoconductivity, persistent absorption) and others (SIMS, EPR).
Date: December 31, 1995
Creator: Becla, P. & Witt, A.F.
Partner: UNT Libraries Government Documents Department

Use of in situ scanning tunneling microscopy for the study of dye sensitization of semiconductor electrodes. Progress report, September 15, 1992--August 15, 1995

Description: In the three years of this contract, the authors have set up a state-of-the-art computer-controlled photoelectrochemical characterization laboratory with facilities to measure Mott-Schottky behavior, photocurrent spectra and photocurrent voltage curves on semiconductor electrodes. They have also set up a Bridgeman crystal growth furnace for preparing their own single crystals of SnS{sub 2} and other semiconductor materials for the photoelectrochemical studies. The first boules of SnS{sub 2} have recently been prepared. They have also modified a scanning tunneling microscope to perform photo-STM experiments to spatially resolve photocurrents on semiconductor surfaces. In addition, the acquisition of a Ti:sapphire laser system from the DOE-URI program will give us the power and flexibility in the light source to provide a better chance of single molecule detection. The results on sensitization will be summarized below.
Date: December 31, 1995
Creator: Parkinson, B.
Partner: UNT Libraries Government Documents Department

LaCl{sub 3}:Ce scintillator for Gamma ray detection

Description: In this paper, we report on a relatively new cerium doped scintillator - LaCl3 for gamma ray spectroscopy. Crystals of this scintillator have been grown using Bridgman method. This material when doped with 10 percent cerium has high light output ({approx} 50,000 photons/MeV) and fast principal decay time constant ({approx}20 ns). Furthermore, it shows excellent energy resolution for gamma ray detection. For example, energy resolution as low as 3.2 percent (FWHM) has been achieved with 662 keV photons (137Cs source) at room temperature. Also high timing resolution (264 ps - FWHM) has been recorded with LaCl3-PMT and BaF2-PMT detectors operating in coincidence using 511 keV positron annihilation gamma ray pairs. Details of crystal growth, scintillation properties, and variation of these properties with cerium concentration are also reported.
Date: May 25, 2002
Creator: Shah, K. S.; Glodo, J.; Klugerman, M.; Cirignano, L.; Moses, W.W.; Derenzo, S.E. et al.
Partner: UNT Libraries Government Documents Department